1. Field of the Invention
The present invention relates to a static eliminator and a static elimination control method, and more specifically to improvement in static eliminator where a drive voltage for corona discharge is repeatedly applied to a discharge electrode, to generate positive ions and negative ions on a periphery of the discharge electrode.
2. Description of Related Art
The static eliminator is a device that supplies positive ions or negative ions to a workpiece charged with static electricity or the like, thereby to remove extra electricity from the workpiece. Static eliminators, which make use of corona discharge that occurs at a tip of a discharging electrode needle at the time of application of a high voltage for drive to the electrode needle, can be grouped into a DC system, an AC system, a pulse DC system, a pulse AC system, and some other type in terms of a drive system at the time of application of the drive voltage to the discharge electrode. In the DC system, a positive-side electrode with a positive potential held with respect to a ground/earth and a negative-side electrode with a negative potential held therewith are provided as discharge electrodes. In the AC system, an AC voltage is applied to a single discharge electrode. In the pulse DC system, a pulse-like drive voltage is alternately applied to a positive-side electrode and a negative-side electrode. In the pulse AC system, a pulse-like AC voltage is applied to a single discharge electrode.
Normally, whether a static elimination object is charged positive or charged negative cannot be seen, and hence it is necessary to generate both positive ions and negative ions. Further, in order to prevent static electricity from remaining on the static elimination object at the time of completion of static elimination, it is necessary to generate the positive ions and the negative ions only in equal amount. However, voltage characteristics of currents flowing through the discharge electrode due to corona discharge in the positive polarity and the negative polarity are not symmetrical (e.g., U.S. Pat. No. 4,872,083). For example, a lower limit of a drive voltage required for corona discharge is lower in the negative polarity. Thereat, in the conventional static eliminator, drive control for the discharge electrode is performed such that positive discharge and negative discharge evenly occur (e.g., Japanese Unexamined Patent Publication No. H8-298197, Japanese Patent No. 4367580, Japanese Unexamined Patent Publication No. 2008-135329, Japanese Patent No. 4219451). Japanese Unexamined Patent Publication No. H8-298197 discloses an AC-system static eliminator, with a discharge electrode applied with a positive DC bias. Japanese Unexamined Patent Publication No. H8-297197 describes adjustment of the DC bias based on an output of an ion sensor.
Japanese Patent No. 4367580 discloses a pulse DC-system static eliminator, which adjusts a ratio between an application time of a positive drive voltage to be applied to the positive-side electrode and an application time of a negative drive voltage to be applied to the negative-side electrode, while holding a drive cycle constant, thereby to adjust ion balance. Japanese Unexamined Patent Publication No. 2008-135329 discloses a pulse AC-type static eliminator, which adjusts respective voltage values of a positive drive voltage and a negative drive voltage, while making an application time of the positive drive voltage remain in agreement with an application time of the negative drive voltage, thereby to adjust ion balance. Further, in Japanese Patent No. 4219451, a ratio between an application time of a positive drive voltage and an application time of a negative drive voltage is adjusted while a drive cycle is held constant, thereby to adjust ion balance.
In the conventional static eliminators described above, it is known that the ion balance changes in accordance with a distance from the static eliminator. Thus, there has been a problem in that, even when the ion balance can be made zero at a specific distance from the static eliminator, the ion balance in the vicinity of the static eliminator significantly deteriorates.